1. Field of the Invention
This invention relates to a porous material of an oxide and/or a complex oxide mainly comprising alumina, zirconia, titania, magnesia, iron oxide or ceria, a process of producing the porous material, a catalyst for purifying exhaust gases comprising the porous material, and a method of purifying exhaust gases.
More specifically, it relates to a porous material having a spongy structure which is suitable for use as a catalyst, a carrier for catalysts, various fillers, a carrier for enzymes, an adsorbent, a filler, and so forth and which is characterized in that (1) the mean pore diameter is in a meso-pore region, (2) the pores have a sharp size distribution, (3) at least a part of the pores form a three-dimensional network structure, and (4) the porous material has substantially no fibrous structure, and a porous material having the above characteristics (1) to (4) which is made up of particles having an aspect ratio of 3 or smaller aggregated together while leaving pores among them; and a process for producing these porous materials.
The present invention also relates to a catalyst and a method for purifying exhaust gases from internal combustion engines of automobiles and the like. More specifically, it relates to a three-way catalyst used for engines run around a stoichiometric air/fuel ratio and a catalyst used for so-called lean-burn engines operated in an oxygen-excess atmosphere. Still more specifically, the invention relates to a three-way catalyst for purifying exhaust gases from conventional engines through simultaneous reduction/oxidation of carbon monoxide (CO), hydrogen (H2), hydrocarbons (HC), and nitrogen oxides (NOx), a catalyst for efficiently reducing nitrogen oxides (NOx) in oxygen-excess exhaust gases which contain oxygen in excess of the amount required to completely oxidize the reducing components, such as carbon monoxide (CO), hydrogen (H2), and hydrocarbons (HC), and a method for purifying exhaust gases.
2. Description of Related Art
The present invention covers the field of a porous material and the field of exhaust gas purification. Disadvantages or drawbacks of related arts are described below separately.
With respect to an alumina porous material having an appropriate pore structure, JP-A-58-190823 and JP-A-60-54917 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d) disclose alumina carriers which have a large pore size with a narrow pore size distribution and yet exhibit excellent mechanical strength.
JP-A-55-27830 teaches a process for producing an alumina porous material having the above-described pore structure, which comprises repeating the steps of adjusting the pH of an aluminum hydroxide slurry to 5 or lower, or 11 or higher and then adjusting the pH to 6 to 10 by addition of a neutralizing agent (a pH swing method). Analogous methods are disclosed in JP-A-58-190823 and JP-A-60-54917.
In regards to a silica porous material having an appropriate pore structure, JP-B-7-64543 (the term xe2x80x9cJP-Bxe2x80x9d as used herein means an xe2x80x9cexamined Japanese patent publicationxe2x80x9d) discloses spherical silica particles having a pore volume of 0.8 to 1.8 ml/g, a surface area of 20 to 500 m2/g, and an average pore size of 80 to 1000 xc3x85. It also teaches a process for preparing the silica porous material comprising drying silica hydrogel, obtained by neutralization of an aqueous alkali silicate solution, with superheated steam at 100 to 1000xc2x0 C. to give silica xerogel. According to the disclosure, it is preferred that the silica hydrogen be previously aged under 0.5 to 5 kg/cm2 of steam for 0.5 to 24 hours.
As for a zirconia porous material, JP-A-8-66631 discloses porous zirconia particles having a sharp pore size distribution that is an important character for use in liquid chromatography, which are obtained by incorporating 0.1 to 10% by weight of silica into zirconium oxide so that the crystal form of zirconium oxide may be prevented from changing during calcination.
With reference to a titania porous material, JP-A-6-340421 proposes needle-like, porous, and fine titanium oxide particles having an average breadth of 80 to 120 xc3x85, an average length of 240 to 500 xc3x85, and an aspect ratio of 2.4 to 6.4, which is produced by a process comprising the steps of (a) allowing a hydrolyzable titanium oxide compound to react with a base to precipitate hydrated titanium oxide, (b) adding a polybasic carboxylic acid to the reaction system to dissolve the hydrated titanium oxide, (c) adding an alkali to the reaction system to hydrolyze the chelated titanium compound, (d) adding an inorganic acid to the precipitate and stirring the system to deflocculate, and (e) dehydrating the resulting fine particles and calcining at 200 to 400xc2x0 C.
Concerning a magnesia porous material, JP-A-59-232915 discloses a process for producing spinel comprising adjusting the pH of a mixed aqueous solution of a water-soluble magnesium salt and a water-soluble aluminum salt with an alkali in the presence of an alcohol to form a precipitate and drying and calcining the precipitate.
As regards an iron oxide porous material, JP-A-61-268358 describes an iron oxide porous material comprising iron oxide and chromium oxide, having a large pore size with a narrow pore size distribution, and exhibiting excellent durability against oxidation and reduction. Similar prior arts are found with respect to a ceria porous material.
According to the above-mentioned pH swing method, which is substantially a method of producing alumina, the pH of boehmite (AlOOH), a precursor, is swung by use of an acidic material and an alkaline material to cause crystals to dissolve and to precipitate alternately thereby letting the crystals grow in a porous fibrous shape with a narrow pore size distribution. However, because the pH should be swung many times, the process is time-consuming and meets difficulty in controlling the conditions for product consistency. Further, when a second component is to be incorporated, it once settles but is then solubilized because of the pH variations, failing to be uniformly dispersed. Or, where a desired second component is such that forms a precipitate at a pH out of a range of from 6 to 11, it is impossible to incorporate the second component into the precursor. Furthermore, the conventional pH swing method does not provide an alumina porous material having a spongy structure nor a porous material comprising an aggregate of particles having an aspect ratio of 3 or smaller.
In particular, the porous materials described in JP-A-58-190823 and JP-A-60-54917 are composed of fibrous particles. When used as a catalyst carrier, a porous material comprising an aggregate of fibrous particles might be capable of supporting a noble metal in a high disperse state. However, as will be explained later in more detail, there will be a certain crystal plane along the fiber length direction so that the catalyst component tends to be supported on that plane in an increased proportion. This helps the catalyst component agglomerate in high temperature.
The spherical silica proposed in JP-B-7-64543 supra is composed of amorphous particles. Where used as a catalyst carrier, it provides no crystal plane to support a noble metal in a high disperse state. It follows that the noble metal particles easily move on the catalyst surface to undergo sintering, resulting in reduction of activity. Silica has lower affinity to noble metal than, for example, alumina, which also contributes to sintering of the supported noble metal particles and reduction of activity. Additionally, where the silica porous material is used in a three-way catalyst, coking occurs to deactivate the catalyst.
None of the aforementioned other prior arts relating to zirconia, titania, magnesia, iron oxide or ceria porous materials proposes a porous material having a spongy structure characterized in that (1) the mean pore diameter is in a meso-pore region, (2) the pores have a sharp size distribution, (3) at least a part of the pores have a three-dimensional network structure, and (4) the porous material has substantially no fibrous structure, or a porous material which is made up of an aggregate of particles having an aspect ratio of 3 or smaller and has the characteristics (1) to (4), still less a process for producing such porous materials.
On the other hand, a three-way catalyst has conventionally been used for treating auto exhaust gases, which catalyzes oxidation of CO and HC and reduction of NOx simultaneously. Conventional three-way catalysts widely known for this particular use comprise, for example, a heat-resistant base material made of, e.g., cordierite, a porous carrier layer of xcex3-alumina formed on the base material, and a noble metal catalyst component, such as platinum or rhodium, supported on the porous carrier layer. A three-way catalyst additionally containing ceria (cerium oxide) having oxygen storage ability to have increased low-temperature activity is also known (see JP-B-6-75675).
However, when these catalysts are exposed to high-temperature exhaust for a long time, the noble metal shows grain growth to reduce its catalytic activity on the simultaneous oxidation-reduction reactions of CO, H2, HC, and NOx in exhaust gases. This is considered to be one of the causes for three-way catalysts to reduce their high-temperature durability.
Carbon dioxide (CO2) in exhaust gases from internal combustion engines of automobiles and the like has now given rise to a serious problem to global environment conservation. A so-called lean-burn engine using a lean fuel mixture is a promising measures against this problem. Lean burn engines use less fuel thereby to suppress CO2 generation.
Since the conventional three-way catalysts aim at simultaneous oxidation of CO and HC and reduction of NOx in exhaust gases at a stoichiometric air/fuel ratio, they are inadequate to reduce NOx in the oxygen-excess atmosphere as in the exhaust gas from lean-burn engines. Therefore, it has been demanded to develop an air cleaning system using a catalyst capable of removing NOx even in an oxygen-excess atmosphere.
Along this line, the present inventors have previously proposed a catalyst for purifying exhaust gases comprising an alkaline earth metal and platinum supported on a porous carrier of alumina, etc. (JP-A-5-317652) and a catalyst for purifying exhaust gases comprising lanthanum and platinum supported on a porous carrier (JP-A-5-168860). In these catalysts the oxide of an alkaline earth metal or lanthanum serves as a NOx storage component under lean conditions, and the stored NOx react with reducing components such as HC, CO, and H2 generated under stoichiometric conditions or in the state of transition from stoichiometric conditions to fuel-rich conditions (at air/fuel ratios lower than the stoichiometric point). Accordingly, they exhibit excellent performance in NOx removal even under lean conditions.
However, an exhaust-gas also contains sulfur oxides (SOx) resulting from combustion of sulfur (S) present in fuel. SOx are oxidized by metallic catalyst components under lean conditions and also react with steam to generate sulfite ions or sulfate ions. The sulfite or sulfate ions can react with the NOx storage component to convert the NOx storage component to its sulfite or sulfate. This phenomenon is called sulfur poisoning. Sulfur poisoning impairs the NOx storing activity of the NOx storage component, which seems to be one of the causes of reduction in NOx removal performance. Upon being heated in a reducing atmosphere, the sulfite or sulfate releases sulfur and returns to its active form. However, if the sulfite or sulfate grows in grains, the sulfur content is hardly released by heating in a reducing atmosphere, and the NOx storing activity is hardly restored.
The recent improvements on engine combustion have made it possible to run lean-burn engines up to a high load, which has further increased the demand for a catalyst for purifying exhaust gases which have higher NOx removal performance. That is, the situation has required that a catalyst for purifying exhaust gases should have high NOx removal performance even in a high-temperature exhaust gas and undergo no reduction in NOx removal performance even when exposed to a high-temperature exhaust gas for a long time (this property will hereinafter be sometimes referred to as high-temperature durability).
However, when a catalyst for purifying exhaust gases is exposed to high-temperature exhaust for a long time, the noble metal shows grain growth to reduce its catalytic activity on the oxidation-reduction reactions. This is considered to be one of the causes of reduction in high-temperature durability of the catalyst.
A first object of the invention is to provide a novel alumina porous material which is amorphous and has a spongy structure characterized in that (1) the mean pore diameter is in a meso-pore region, (2) the pores have a sharp size distribution, (3) at least a part of the pores have a three-dimensional network structure, and (4) the porous material has substantially no fibrous structure, i.e., a spongy and porous structure of alumina whose pore sizes are highly concentrated in the vicinity of the mean pore diameter.
A second object of the invention is to provide a novel porous material made up of aggregated particles having an aspect ratio of 3 or smaller which has the above-described characteristics (1) through (4), i.e., a porous material wherein the particles have an aspect ratio of 3 or smaller and aggregate together to form pores among the particles and whose pore sizes are highly concentrated in the vicinity of the mean pore diameter.
A third object of the invention is to provide a process for producing the above-mentioned porous materials easily and economically.
A fourth object of the invention is to provide a process for producing a porous material, which process permits addition of a second component for improving the quality of the porous material.
A fifth object of the invention is to provide a catalyst excellent in exhaust gas purifying ability. More particularly, the object is to suppress grain growth of a noble metal which occurs in a high-temperature exhaust gas and/or grain growth of sulfites or sulfates produced by the reaction between SOx and an NOx storage component, thereby to provide a catalyst for exhaust gas purification which is excellent in high-temperature durability, NOx removal performance, and resistance against sulfur poisoning.
A sixth object of the invention is to provide a method of purifying exhaust gases using the above-described catalyst.
The first and second objects of the invention are accomplished by a porous material comprising particles without substantial fibrous structure and having pores, the pores having a mean pore diameter in a meso-pore region, a sharp pore size distribution, and at least a part of the pores being connected three-dimensionally to form a three-dimensional network structure with random passages. The mean pore diameter preferably is in a range of from 2 to 100 nm. There is thus provided a novel porous material suited for use as a catalyst, a carrier for catalysts, various filters, a carrier for enzymes, an adsorbent, a filler, and so forth.
The porous material of the present invention is, in its first aspect, a porous material wherein said particles are alumina, and the three-dimensional network structure has a spongy structure. It is preferred that this porous material be amorphous and the pores exist on the surface and in the inside thereof. The first aspect provides a novel porous material satisfying the first object of the invention and useful in the above-described applications.
The porous material according to the first aspect includes:
(1) an embodiment in which the volume of the pores within xc2x15 nm at the mean pore diameter of the total meso-pore volume occupies 70% or more,
(2) an embodiment in which the volume of the pores within xc2x15 nm at the mean pore diameter of the total meso-pore volume occupies 80% or more, or the volume of the pores within xc2x13 nm at the mean pore diameter of the total meso-pore volume occupies 70% or more,
(3) an embodiment in which the volume of the pores within xc2x15 nm at the mean pore diameter of the total meso-pore volume occupies 90% or more, or the volume of the pores within xc2x13 nm at the mean pore diameter of the total meso-pore volume occupies 80% or more, and
(4) an embodiment in which the volume of the pores within xc2x13 nm at the mean pore diameter of the total meso-pore volume occupies 90% or more.
Having such a sharp pore size distribution, the porous material is particularly useful in. reaction, separation, adsorption, desorption or the like operation in which molecules take part as hereinafter described in detail.
The porous material according to the first aspect is obtained by precipitating at least a part of an aluminum component from an aluminum salt aqueous solution at a pH 3 to 4.5, aging the aqueous solution as containing the precipitate in saturated vapor or nearly saturated vapor for a prescribed period of time to produce a precursor, and calcining the precursor. It is preferred that the calcination of the precursor is to remove water to make amorphous while retaining the grain arrangement of the precursor.
The porous material according to the invention is, in its second aspect, a porous material wherein the particles have an aspect ratio of 3 or smaller and aggregate together to form pores among the particles. It is preferred that the particles are crystalline oxides in which said crystalline oxide particles are connected three-dimensionally to form pores among the particles. The second aspect provides a novel porous material which meets the second object of the invention and is suited for use in the above-described applications.
Specifically, the porous material according to the second aspect comprises at least one of crystalline oxides and crystalline complex oxides selected from the group consisting of alumina, zirconia, titania, magnesia, iron oxide, and ceria.
The alumina porous material of the second aspect includes:
(1) an embodiment in which the volume of the pores within xc2x15 nm at the mean pore diameter of the total meso-pore volume occupies 70% or more,
(2) an embodiment in which the volume of the pores within xc2x15 nm at the mean pore diameter of the total meso-pore volume occupies 80% or more, or the volume of the pores within xc2x13 nm at the mean pore diameter of the total meso-pore volume occupies 70% or more,
(3) an embodiment in which the volume of the pores within xc2x15 nm at the mean pore diameter of the total meso-pore volume occupies 90% or more, or the volume of the pores within xc2x13 nm at the mean pore diameter of the total meso-pore volume occupies 80% or more, and
(4) an embodiment in which the volume of the pores within xc2x13 nm at the mean pore diameter of the total meso-pore volume occupies 90% or more.
Having such a sharp pore size distribution, the porous alumina material is particularly useful in reaction, separation, adsorption, desorption or the like operation in which molecules take part as hereinafter described in detail.
The porous material of the second aspect comprising other oxides than alumina includes:
(5) a zirconia porous material in which the volume of the pores within xc2x15 nm at the mean pore diameter of the total meso-pore volume occupies 40% or more,
(6) a titania porous material in which the volume of the pores within xc2x15 nm at the mean pore diameter of the total mesb-pore volume occupies 50% or more, or the volume of the pores within xc2x13 nm at the mean pore diameter of the total meso-pore volume occupies 40% or more,
(7) a magnesia porous material in which the volume of the pores within xc2x15 nm at the mean pore diameter of the total meso-pore volume occupies 80% or more,
(8) an iron oxide porous material in which the volume of the pores within xc2x15 nm at the mean pore diameter of the total meso-pore volume occupies 40% or more, and
(9) a ceria porous material in which the volume of the pores within xc2x15 nm at the mean pore diameter of the total meso-pore volume occupies 70% or more, or the volume of the pores within xc2x13 nm at the mean pore diameter of the total meso-pore volume occupies 55% or more.
Similarly to the alumina porous material, the porous materials of oxides other than alumina have such a sharp pore size distribution and are useful in reaction, separation, adsorption, desorption or the like operation in which molecules take part.
The porous material according to the second aspect is obtained by aging in saturated vapor or nearly saturated vapor for a prescribed period of time a system capable of becoming an oxide on thermal decomposition to produce a precursor and calcining the precursor. It is preferred that the calcination of the precursor is to remove solvent to make an oxide while retaining the grain arrangement of the precursor.
It is preferred for the alumina porous material of the second aspect to contain at least one element selected from rare earth elements, alkaline earth metals, and group IV elements as an additive component. It is particularly preferred to add lanthanum as a rare earth element and/or at least one of titanium, silicon and zirconium as a group IV element. Such a second component added brings about improved heat resistance or arbitrarily modifies various characteristics, such as acidity or basicity, according to the end use and is particularly effective in preparing a catalyst for purifying exhaust gases or a catalyst for modifying fuel.
The third and fourth objects of the invention are accomplished by a process for producing a porous material comprising the steps of:
(1) preparing a system capable of becoming an oxide on thermal decomposition,
(2) aging the system at or above room temperature for a prescribed time to form a precursor, and
(3) calcining the precursor.
According to this process the porous material according to the invention can be produced easily and economically. In particular, where it is desired to add a second component to the porous material according to the second aspect to improve or modify the quality of the porous material, the process easily permits such addition.
Step (1) preferably includes embodiments, wherein:
(a) the system contains a solvent,
(b) the system is a solvated system,
(c) the solvent is water, a monohydric alcohol, a dihydric alcohol, a trihydric alcohol, or a mixture of one or more thereof,
(d) the solvent is water, a monohydric alcohol or a mixture thereof,
(e) the system consists of at least one of a hydroxide of aluminum, zirconium, titanium, magnesium, iron or cerium and a salt of aluminum, zirconium, titanium, magnesium, iron or cerium,
(f) the system comprises at least one of a hydroxide of aluminum, zirconium, titanium, magnesium, iron or cerium and a salt of aluminum, zirconium, titanium, magnesium, iron or cerium as a main component, and optionally at least one of a rare earth element, an alkaline earth metal, and a group IV metal as an additive component,
(g) the above-enumerated hydroxide or salt is immersed in a solvent, and
(h) the system is a precipitate immersed in a solvent, the precipitate being formed from an aluminum salt, a zirconium salt, a titanium salt, a magnesium salt, an iron salt or a cerium salt.
Step (2), an aging step, is preferably carried out by aging the system prepared in step (1) in saturated vapor (preferably steam) or nearly saturated vapor for a prescribed period of time to form a precursor. Where steam is used, the temperature of the steam is preferably 200xc2x0 C. or lower, still preferably 80 to 150xc2x0 C., particularly preferably 100 to 130xc2x0 C. The aging time is usually from 0-5 to 200 hours.
Step (3), a calcining step, is preferably carried out by removing the solvent while retaining the grain arrangement of the precursor. The calcining temperature, while dependent on the kind of the desired porous material, usually ranges from 300 to 1200xc2x0 C.
The fifth object of the invention, particularly suppressing grain growth of a noble metal which occurs in a high-temperature exhaust gas, is achieved by a catalyst for purifying exhaust gases which comprises, in its first aspect, a carrier and a noble metal supported on the carrier, in which at least a part of the carrier comprises the porous material according to the invention (inclusive of the first and the second aspects of the porous material).
As stated above, since the porous material according to the invention comprises particles without substantial fibrous structure and has pores having (1) a mean pore diameter in a meso-pore region, (2) sharp pore size distribution, and (3) at least a part of the pores being connected three-dimensionally to form a three-dimensional network structure with random passages, the following actions and effects are assumed.
Because of the characteristic (1), a noble metal can be supported stably and in a highly disperse state. Owing to the characteristic (2), a noble metal can be supported uniformly, starting points of grain growth hardly generate, and grain growth of the noble metal in high temperature can be restrained.
By virtue of the characteristics (1) and (3), unburnt HC, CO, NOx, etc. in an exhaust gas passing through the pores are diffused throughout the catalyst while reacting efficiently.
Further, since the porous material according to the present invention comprises particles without substantial fibrous structure, this characteristic secures prevention of the noble metal from agglomeration and grain growth thereby to provide a catalyst for purifying exhaust gases excellent in high-temperature durability. If the particles making up the carrier is fibrous, there is a certain crystal plane that is along the fiber direction. As a result, a noble metal, even though supported in a highly disperse state, will be supported on the same crystal plane in an increased proportion and therefore tend to agglomerate as compared with one supported on different crystal planes.
As compared with such a fibrous carrier, since the porous material according to the second aspect is made up of particles having an aspect ratio of 3 or less, the crystal planes are limited by the particle size so that the crystal planes on which a noble metal is supported are limited. As a result, the noble metal particles hardly agglomerate among themselves, and grain growth of the noble metal in high temperature can be hindered.
In case where the particles constituting a carrier is amorphous, lack of a crystal plane (on which a noble metal can be held) allows noble metal particles to move easily and agglomerate on the surface of the carrier even though the particles could be supported in a highly disperse state. In contrast, although the porous material according to the first aspect is made up of amorphous particles, it has a spongy structure with pores being made of recesses, so that noble metal particles can be held in the pores stably, being prevented from agglomerating among themselves and growing in grains in high temperature.
Thus, use of the porous material according to the invention as a carrier provides a catalyst which is particularly excellent in exhaust gas purifying ability and high-temperature durability.
In a preferred embodiment, the catalyst according to the first aspect has the noble metal concentrated in the vicinity of the carrier surface. Thus, a well-known method of depositing a noble metal on a carrier can be applied to the porous material of the invention to have the noble metal supported on the surface of the porous material in a high concentration thereby providing a catalyst excellent in exhaust gas purification performance in which grain growth of a noble metal is suppressed.
In order to accomplish the fifth object of the invention, particularly in order to suppress grain growth of sulfites or sulfates generated by the reaction between SOx and an NOx storage component and/or grain growth of a noble metal which occurs particularly in a high-temperature exhaust gas thereby to provide a catalyst for purifying exhaust gases having high NOx removal performance, high resistance against sulfur poisoning, and high high-temperature durability, the present inventors have extensively studied on a catalyst comprising a carrier, an NOx storage component comprising at least one of an alkali metal, an alkaline earth metal, and a rare earth element that is supported on the carrier, and a noble metal supported on the carrier. As a result, they have surprisingly found that use of the porous material having a specific structure according to the invention as a carrier provides a catalyst having excellent NOx removal performance, high sulfur poisoning resistance and satisfactory high-temperature durability and a method of purifying exhaust gases.
Accordingly, the catalyst for purifying exhaust gases according to the second aspect comprises a carrier, an NOx storage component comprising at least one of an alkali metal, an alkaline earth metal, and a rare earth element that is supported on the carrier, and a noble metal supported on the carrier, in which at least a part of the carrier comprises the porous material of the invention (inclusive of the porous material according to the first aspect and that of the second aspect).
The structure and the mechanism of actions and effects of the catalyst according to the second aspect will be described below in detail. Note that the description about the mechanism of action contains some unexamined assumptions so that whether it is right or wrong is not deemed to restrict the invention.
The porous material, which constitutes at least a part of the carrier, is characterized by comprising particles without substantial fibrous structure and having pores, the pores having a mean pore diameter of the pores being in a range of 2 to 100 nm, sharp pore size distribution, at least a part of the pores being connected three-dimensionally to form a three-dimensional network structure with random passages.
It is assumed that these characteristics of the carrier have the following actions.
To have a mean pore diameter of 2 nm or greater secures adsorption of the NOx storage component and the noble metal onto the carrier. Further, the catalyst can taken in exhaust gas components for certain without suffering from clogging of the pores, thereby achieving exhaust gas purification through the oxidation-reduction reactions with certainty.
To have a mean pore diameter of 100 nm or smaller is effective at preventing sulfite or sulfate grains produced by the reaction between SOx in an exhaust gas and the NOx storage component from growing and in confining them to the size of the pores. As a result, the sulfites or sulfates are ready to decompose on shifting the air/fuel ratio from a stoichiometric point to a fuel-rich side. Thus, the NOx storage component can be prevented from reducing its NOx storage ability.
To have a mean pore diameter of 100 nm or smaller also produces an effect in suppressing grain growth of a noble metal that is liable to occur particularly in a high-temperature exhaust gas and thereby minimizing the reduction in catalytic activity.
Where at least a part of the pores are connected three-dimensionally to form random passages, the passages forming a three-dimensional network structure, the noble metal serving for catalysis can be supported on the carrier stably and in a high disperse state.
In case where a porous material has a fibrous structure, a noble metal is apt to be supported on a crystal plane that is present along the fiber direction. A noble metal on a fibrous porous material, even if highly dispersed, tends to grow in grain in high temperature. Since the porous material used as a carrier of the catalyst according to the second aspect has substantially no fibrous structure, grain growth of the noble metal attributed to a fibrous structure does not take place so that the noble metal can be protected from reduction of activity even in high temperature.
Based on these actions, there is provided a catalyst for purifying exhaust gases having high NOx removal performance, high sulfur poisoning resistance, and excellent high-temperature durability, that is, a catalyst that retains the NOx storage ability of the NOx storage component and the catalytic activity of the noble metal with certainty while suppressing grain growth of sulfites or sulfates generated by the reaction between SOx and an NOx storage component and/or grain growth of the noble metal which occurs particularly in a high-temperature exhaust gas.
In a first embodiment of the catalyst according to the second aspect, the pores of the carrier are those formed by aggregation of particles having an aspect ratio of 3 or less (the carrier used here corresponds to the porous material according to the second aspect). In this embodiment, a noble metal, a catalyst component, can be supported stably and in a highly disperse state on the carrier thereby to provide a catalyst with high NOx removing ability.
In this first embodiment, the particles constituting the porous carrier are preferably crystalline particles of an oxide. In this case, the carrier has crystal planes, and the noble metal, hardly moving on the carrier surface, is inhibited from agglomerating. There is thus provided a catalyst for purifying exhaust gases exhibiting particularly high NOx removal performance.
The oxide is an oxide and/or a complex oxide selected from the group consisting of alumina, zirconia, titania, magnesia, iron oxide, and ceria. Any of the above-described oxides can be used as particles constituting the porous carrier, there are provided catalysts which take advantage of alumina, zirconia, titania, magnesia, iron oxide or ceria.
The porous material can contain at least one of a rare earth element, an alkaline earth metal, and a group IV element as an additive component so as to have improved heat resistance or modified characteristics. Naturally, the heat resistance or various characteristics of the resulting catalyst are also improved or modified. In particular, addition of lanthanum as a rare earth element brings about improvement in heat resistance to provide a highly heat-resistant catalyst for purifying exhaust gases.
In a second embodiment of the catalyst according to the second aspect, the porous material as a carrier is of alumina, and the alumina porous material has a spongy structure and is amorphous (the carrier used here corresponds to the porous material according to the first aspect). Because the alumina porous material is amorphous and yet has a spongy structure, the pores are made of recesses in which noble metal grains can be settled stably, being prevented from growing in grains. The catalyst therefore maintains high NOx removal performance.
In a third embodiment of the catalyst according to the second aspect, the porous material used as a carrier has such a pore size distribution that the volume of the pores whose diameter is in a range of from 2 to 20 nm in the total volume of the pores whose diameter is in a range of from 2 to 100 nm occupies 70% or more (the carrier used here corresponds to the porous material of the invention, inclusive of the first and second aspects thereof) In this embodiment, sulfites or sulfates produced by the reaction between SOx present in exhaust gas and the NOx storage component are inhibited from grain growth and confined in size with certainty. As a result, the sulfites and sulfates are ready to decompose so that reduction of the NOx storage ability of the NOx storage component can be minimized. Also, the noble metal can retain its catalytic activity because it is suppressed from grain growth particularly in a high temperature exhaust gas with certainty. There is thus obtained a catalyst for purifying exhaust gases superior in NOx removal performance, resistance to sulfur poisoning, and high-temperature durability.
The method for purifying exhaust gases which accomplishes the sixth object of the invention comprises setting the catalyst having the aforementioned actions according to the second aspect in an oxygen-excess exhaust gas (air/fuel weight ratio of 18 or higher) to store the NOx in the NOx storage component from the exhaust gas containing NOx, making the stored NOx released from the NOx storage component and reducing the released NOx by periodically shifting the air/fuel weight ratio of an engine from a stoichiometric point to a fuel excess side, and, at the same time, decomposing sulfites or sulfates.
The exhaust gas purifying method of the invention is capable of accelerating the decomposition of sulfites or sulfates produced by the reaction between SOx and the NOx storage component while certainly retaining the NOx storage ability of the NOx storage component and the catalytic activity of the noble metal. Therefore, the method achieves high performance in NOx removal and high resistance against sulfur poisoning.